1. Field of the Invention
The present invention relates to a guide bush mounted on an automatic lathe to support a rodlike workpiece for rotation and axial sliding at a position near a cutting tool (cutter), and a method of forming a hard carbon film over the inner surface of the guide bush to be in sliding contact with the workpiece.
2. Description of the Related Art
Guide bushes mounted on the column of an automatic lathe to hold a rodlike workpiece for rotation at a position near a cutting tool are classified into rotary guide bushes and stationary guide bushes. A rotary guide bush rotates together with a workpiece and holds the workpiece for axial sliding. A stationary guide bush remains stationary and holds a workpiece for rotation and axial sliding.
A guide bush of either type has a portion having a taper outer surface provided with slits to make the same portion elastic, a threaded portion to hold the guide bush on the column, and an inner surface for holding a workpiece. The inner surface always in sliding contact with a workpiece is liable to be worn and, particularly, the inner surface of a stationary guide bush is worn rapidly.
A guide bush proposed in JP-A No. 4-141303 has an inner surface to be in sliding contact with a workpiece, which slides and rotates on the inner surface, attached with a superhard alloy or a ceramic material attached to the inner surface by brazing or the like.
When the inner surface of a guide bush is attached with a superhard alloy or a ceramic material excellent in wear resistance and heat resistance, the wear of the inner surface of the guide bush can be reduced to some extent.
However, when the workpiece is subjected to heavy machining on an automatic lathe, in which the depth of cut is large and the cutting speed is high, the workpiece is damaged or seizing occurs due to decrease in the diametrical clearance between the guide bush and the workpiece even if the inner surface of the guide bush is attached with a superhard alloy or a ceramic material, because the superhard alloy and the ceramic material have a comparatively large coefficient of friction and a low thermal conductivity. Therefore, it has been difficult to increase the depth of cut and cutting speed.
The stationary guide bush has advantages that a workpiece can be accurately machined in a high roundness because the workpiece can be held so that its axis may not run out, less noise is generated, and the automatic lathe may be of a simple, compact construction.
However, the inner surface of the stationary guide bush is worn far more rapidly than that of the rotary guide bush and hence it is more difficult to increase depth of cut and cutting speed when the stationary guide bush is employed.
In order to solve the problem, the inventors have proposed to form a hard carbon film over the inner surface of a guide bush slidably contacting with a workpiece, which can drastically improve abrasion resistance of the inner surface of a guide bush without occurrence of damage to a workpiece nor seizing, and which can increase the cutting amount and machining speed of an automatic lathe.
The hard carbon film is formed of a hydrogenated amorphous carbon closely resembling diamond in properties. Therefore, hydrogenated amorphous carbon is also called diamond like carbon (DLC).
The hard carbon film (DLC film) has high hardness (not lower than Vickers 3000 Hv), is excellent in wear resistance and corrosion resistance, and has a small coefficient of friction (about 1/8 of a superhard alloy).
There is a plasma CVD method (plasma Chemical Vapor Deposition Process) for forming a hard carbon film as a method of forming the aforementioned hard carbon film on the inner surface of the guide bush, in which, for example, a plasma is produced by evacuating a vessel to a vacuum of 5.times.10.sup.-3 torr at which the film is formed in an atmosphere of a gas containing carbon and a DC voltage of -3 kV is applied to the guide bush by a DC power source.
However, since the hard carbon film is formed by decomposing the gas containing carbon mainly by the plasma produced around a region surrounding the guide bush in the plasma CVD method, the hard carbon film is formed uniformly on the outer surface of the guide bush, but the hard carbon film formed on the inner surface of the guide bush is inferior in adhesion and moreover inferior in film qualities, such as the hardness.
This is because the plasma in the center bore of the guide bush generates an abnormal discharge called a hollow discharge since electrodes of the same potential face each other in the space of the center bore. The hard carbon film formed by this hollow discharge is a polymer-like film which is inferior in adhesion and rigidity, and is easily peeled off the inner surface of the guide bush.
In the aforementioned method of forming a hard carbon film, the guide bush is applied with a DC voltage of -3 kV by a DC voltage power source at a vacuum of 5.times.10.sup.-3 torr at which the film is formed.
In such a state of a vacuum of 5.times.10.sup.-3 torr in the vacuum vessel, electric charges are liable to be concentrated in the space of the vacuum vessel, resulting in low impedance in the space. Therefore, an abnormal discharge, i.e., an arc discharge is liable to be caused at an instant when the plasma discharge starts.
Further, adhesion of the film to the guide bush depends on the film quality formed at this initial stage of forming the film because the time when the plasma discharge starts is the initial stage of forming the hard carbon film.
Accordingly, there arises a problem that the quality and adhesion of the hard carbon film is lowered and the hard carbon film is peeled off the inner surface of the guide bush when the abnormal discharge, i.e., the arc discharge, is generated at the initial stage of the plasma discharge.
Accordingly, the present invention is intended to solve such problems and to form a hard carbon film on the inner surface of a guide bush slidably contacting a workpiece with excellent quality and adhesion.